Re: std::__debug::vector default ctor must not be defaulted

[Stephan Bergmann <sbergman at redhat dot com>]

On 04/28/2015 07:25 PM, Jonathan Wakely wrote:
> On 28/04/15 17:42 +0200, Stephan Bergmann wrote:
> > I think
> >
> > > #if __cplusplus < 201103L
> > >      vector() _GLIBCXX_NOEXCEPT
> > >      : _Base() { }
> > > #else
> > >      vector() = default;
> > > #endif
> >
> > (libstdc++-v3/include/debug/vector) introduced with r210123 (present
> > in GCC 5 and trunk) is wrong.  The standard mandates that the
> > std::vector default ctor is user-provided ([vector.overview] "vector()
> > noexcept : vector(Allocator()) { }"), and defaulting it breaks code like
> >
> >  const std::vector< double > aDXArray;
> >
> > because of [dcl.init] "If a program calls for the default
> > initialization of an object of a const-qualified type T, T shall be a
> > class type with a user-provided default constructor." (as flagged at
> > least by trunk Clang).
>
> See the notes about DR 253 at https://gcc.gnu.org/gcc-4.6/changes.html
> which explains why GCC accepts it. The expectation is that Clang will
> start accepting it at some point too.
>
> I'm not convinced it's worth changing anything here.
>
> The main advantage of defaulting the constructor is that we don't have
> to restate the exception specification on the debug vector.

Ah, hadn't been aware of DR 253.  Makes sense.

> The disadvantage is that you can't compile almost useless code (an
> empty vector that can never be modified to be non-empty) without
> adding an explicit initializer.

Turns out there are three places in LibreOffice that use such empty 
const vectors (to pass into a function that takes a vector const& 
argument, or to verbosely return an empty vector from a function, 
[1][2]), but of course no problem to put explicit initializers there.

[1] 
<http://cgit.freedesktop.org/libreoffice/core/commit/?id=aef1826d9ca318a1359a2116e7f146e993413234>
[2] 
<http://cgit.freedesktop.org/libreoffice/core/commit/?id=aef1826d9ca318a1359a2116e7f146e993413234>